In the manufacture of vehicular instruments such as pressure and temperature gauges, it has been common practice to provide gauge housings or casings that are completely sealed so that the interior of the housing may be evacuated and filled with an inert gas such as argon. This evacuation, filling and sealing process frees the gauge mechanism from the entry of foreign material and also permits the instrument, when used as a pressure gauge, to more accurately indicate "absolute" pressure as opposed to "gauge" pressure.
The evacuation and filling process is usually performed after the gauge is completely assembled and sealed with the exception of a filling hole in the casing. This filling hole is generally used both in evacuating air from the instrument and also, immediately thereafter, in filling the housing with the inert gas. Thereafter the filling hole is sealed by a plug or fastener of some type.
Many vehicular instruments have pressure responsive elements in the operating mechanisms for driving the indicator. These include pressure responsive diaphragms and bourdon tubes. In diaphragm type movements a circular flexible diaphragm encloses one side of a pressure chamber which is exposed to varying pressures representing the variable parameter to be indicated. As pressure in the chamber increases, the diaphragm flexes outwardly moving an actuator element a distance proportional to the pressure within the chamber. A suitable mechanism is provided for converting the axial movement of this actuator to proportional pivotal movement of an indicator needle. In bourdon tube instruments, a curved tube, generally elliptical in cross-section, is connected directly to the indicator needle and in response to increasing pressures, the tube changes cross-section and elongates directly driving the pivotal indicating needle usually without any intermediate mechanism.
In both the diaphragm type and bourdon tube type instruments, there are instances when the components sealing the pressure chambers fail causing a rapid pressure increase within the housing of the instrument. Since these instruments are tightly sealed this creates the possibility of a very high pressure within the casing, possibly causing a rupture in the casing or more commonly in the lens closing the forward face of the casing, which is in the direction of the operator of the associated vehicle. Such instances, while infrequent, may cause serious injury to the operator of the vehicle, and less importantly, damage to other parts of the vehicle.
It is a primary object of the present invention to ameliorate the problems noted above and to provide an improved pressure relief valve and seal for a sealed casing instrument of the type that responds to variations in fluid pressure.